This invention relates to methods for joining two conductive sheets by welding.
There are many situations in which two sheets of metal or, more generally, two thin electrically conductive sheets are to be joined by welding. The increasing emphasis on reliability to reduce warranty costs in consumer products or to meet stringent reliability requirements in high-tech and military applications requires great consistency from weld to weld.
One method of welding together two sheets of electrically conductive material is to juxtapose the sheets by placing one sheet over the other in the region to be welded, and then sandwiching the sheets between a pair of welding electrodes. A high current is then passed through the electrodes, through the sheets of metal and their interface to thereby heat the metal near the interface to a fusion or welding temperature. In some cases, as when welding a bond wire or interconnection cable to a metallized surface of a semiconductor, it is not possible to apply an electrode to the bottom surface of the two metal sheets to be welded. Other methods may be used for semiconductor welding, as for example, rapidly vibrating one of the pieces to be joined against the other, to thereby generate fusing heat at the moving interface and to concurrently scrape the area clear of oxides and dirt.
Japanese Patent Document No. 56-160885 describes a system for welding semiconductor elements on a substrate including a pair of welding electrodes and means for metering optimum contact resistance between the electrodes and the parts to be welded prior to welding, for achieving optimum contact resistance between the electrodes and the part despite the existence of oxidized films or other foreign material on the parts. A high reliability method is desired for welding together two sheets of conductive material accessible for welding from only one side.